Illustration of the structure of the spike (S) protein from the new coronavirus SARS-CoV-2 (previously 2019-CoV). S proteins are found in the viral membrane, they bind to receptors on host cell membranes and facilitate the virus's entry to the cell. Once inside the cell, the virus uses the cells' machinery to make more copies of itself. SARS-CoV-2 emerged in Wuhan, China, in December 2019. The virus causes the respiratory disease Covid-19 that can develop into pneumonia and be fatal in some cases.